There are 3 significant things I can think of that affect the impedance of
trace features on 2 sided micro strip boards. Substrate dielectric constant,
substrate thickness, and copper conductor trace width. These types of boards
are typically single conductor traces over a ground plane structure.

Variation in dielectric thickness and trace width are typically the
manufacturing process related variables.  A thousandth of an inch difference
in trace width or in dielectric thickness can have a large affect on the
impedance of a very thin line at higher frequencies... and getting more and
more sensitive as frequency increases. We had to use special shops that had
very good etching controls in their processes in order to use printed
elements in our Microwave boards, like printed mixers, couplers, power
splitters, edge coupled filters, etc. A small change in trace width could
move the performance of the board out of tolerance electrically.

We chose trace widths that would be less affected by variation in the
geometries. The percentage change in trace width was the key factor in
repeatable performance from board to board. At 6GHz we used 22 mil traces on
.040 thick material (if I remember correctly) and chose this width because
manufacturers can keep the trace width to within a small percentage of the
nominal size.


Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510

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